1. Field of the Invention
The invention relates to a thin film semi-conductor device and, more particularly, to a thin film semiconductor device which can be also suitably used as a photoelectric converting device which can be used in an image processing apparatus such as facsimile, digital copying apparatus, image reader, or the like.
2. Related Background Art
A thin film semiconductor made of a non-monocrystalline semiconductor, particularly, non-monocrystalline silicon (polysilicon, crystallite silicon, and amorphous silicon) is suitably used as a photoelectric converting device which can be preferably used as a thin film semiconductor device in a photoelectric converting device having a large area or a long length. As a photoelectric converting device using a thin film semiconductor, there are two kinds of devices such as primary photo-current type (photodiode type) device and secondary photo current type device. Although the primary photo-current type is a photoelectric converting device to extract electrons and holes generated by the incident light and photoelectrically converted, there is a problem such that a photo current which can be taken as an output is small. On the other hand, according to the secondary photo current type photoelectric converting device, since a larger photo current (secondary photo current) can be obtained as compared with that of the primary photo current type photoelectric converting device, it can be applied to apparatuses in a wider range and attention is paid to it.
FIG. 1 is a schematic constructional diagram for explaining an example of the secondary photo current type photoelectric converting device. In FIG. 1, reference numeral 1011 denotes an insulative substrate such as glass or the like; 1012 a photoconductive semiconductor layer made of CdS-Se, amorphous silicon hydride (hereinafter, abbreviated to "a-Si:H") formed by a plasma CVD method or the like, etc.; 1013a and 1013b impurity layers for ohmic contact; and 1014a and 1014b electrodes. In the above construction, by applying a voltage across the electrodes 1014a and 1014b, a large secondary photo current flows and is photoelectrically converted when the light enters from the side of the substrate 1011 or the side of the electrodes 1014a and 1014b.
Further, a photoelectric converting device the thin film transistor type having auxiliary electrodes to stabilize and improve the characteristics (photo current, dark current, etc.) is proposed. FIG. 2 is a schematic constructional diagram of a thin film transistor type photoelectric converting device having auxiliary electrodes. In FIG. 2, the same component elements as those shown in FIG. 1 are designated by the same reference numerals. In FIG. 2, reference numeral 1015 denotes a transparent or opaque gate electrode and 1016 indicates a gate insulative layer made of SiN.sub.x or the like and formed by a plasma CVD method or the like.
Further, a complete contact type photo sensor (photoelectric converting device) using the thin film transistor type photoelectric converting device of FIG. 2, a thin film transistor, and the like is proposed (JP-A-61-26365). FIG. 3 shows an example of such a circuit. FIG. 3 relates to the case of a sensor array having nine thin film transistor type photoelectric converting sections. In the diagram, one block is constructed by every three of thin film transistor type photoelectric converting sections E.sub.1 to E.sub.9. Thus, three blocks are formed by the converting sections E.sub.1 to E.sub.9. The sensor array is constructed by those three blocks. Capacitors C.sub.1 to C.sub.9 and switching transistors T.sub.1 to T.sub.9 are respectively connected to the converting sections E.sub.1 to E.sub.9 in correspondence thereto. Individual electrodes of the photoelectric converting sections E.sub.1 to E.sub.9 are connected to corresponding proper one of common lines 3102 to 3104 through the switching transitors T.sub.1 to T.sub.9. In more detail, the first switching transistors T.sub.1, T.sub.4, and T.sub.7 of each block are connected to the common line 3102. The second switching transistors T.sub.2, T.sub.5, and T.sub.8 of each block are connected to the common line 3103. The third switching transistors T.sub.3, T.sub.6, and T.sub.9 of each block are connected to the common line 3104. The common lines 3102 to 3104 are connected to an amplifier 3105 through switching transistors T.sub.10 to T.sub.12.
Gate electrodes of switching transistors ST.sub.1 to ST.sub.9 are commonly connected every block in a manner similar to the gate electrodes of the switching transistors T.sub.1 to T.sub.9 and are connected to parallel output terminals of a shift register 3109 every block. Therefore, the switching transistors ST.sub.1 to ST.sub.9 are sequentially turned on every block by a shift timing of the shift register 3109.
In FIG. 3, the common lines 3102 to 3104 are respectively connected to the ground through capacitors C.sub.10 to C.sub.12 and to the ground through switching transistors CT.sub.1 to CT.sub.3. A capacitance of each of the capacitors C.sub.10 to C.sub.12 is set to be sufficiently larger than that of each of the capacitors C.sub.1 to C.sub.9. Gate electrodes of the switching transistors CT.sub.1 to CT.sub.3 are commonly connected and are also connected to a terminal 3108. That is, by applying a high level signal to the terminal 3108, the switching transistors CT.sub.1 to CT.sub.3 are simultaneously turned on, so that the common-lines 3102 to 3104 are connected to the ground. Further, the photoelectric converting sections E.sub.1 to E.sub.9 have gate electrodes G.sub.1 to G.sub.9.
FIG. 4 is a partial plan view of a one-dimensional complete contact sensor array formed on the basis of the circuit diagram shown in FIG. 3. In the diagram, reference numeral 3111 denotes a matrix-shaped wiring portion comprising the common lines 3102 to 3104 and the like; 3112 indicates a thin film transistor type photoelectric converting section; 3113 a charge accumulating section comprising the capacitors C.sub.1 to C.sub.9 ; 3114 a transfer switch which is constructed by the switching transistor T.sub.1 to T.sub.9 and uses thin film transistors having the same structure as that of the photoelectric converting section; 3115 a discharge switch which comprises the switching transistors ST.sub.1 to ST.sub.9 and uses thin film transistors having the same structure as that of the photoelectric converting section; 3116 an extension line to connect a signal output of the transfer switch 3114 to a signal processing IC: and 3117 a load capacitor which comprises the capacitors CT.sub.1 to CT.sub.3 and is used to accumulate the signal charges which have been transferred by the transfer switch 3114 and to read out the signal charges.
FIG. 5 is a cross sectional view taken along the line A--A' in FIG. 4. As will be obviously understood from FIG. 5, all of the thin film transistor type photoelectric converting section 3112, charge accumulating section 3113, transfer switch 3114, discharge switch 3115, matrix-shaped wiring section 3111, load capacitor 3117, and the like have a common construction in which a metal (gate electrode 1015 in the photoelectric converting section), an insulative layer (gate insulative layer 1016 in the photoelectric converting section), a photoconductive semiconductor layer (1012 in the photoelectric converting section), ohmic contact layers (1013a and 1013b in the photoelectric converting section), and metals (1014a and 1014b in the photoelectric converting section) are formed on the substrate 1011 in accordance with this order.
The sensor array shown in FIGS. 4 and 5 as mentioned above has the con, non construction in order to reduce the manufacturing costs by simultaneously manufacturing the photoelectric converting section and the drive circuit section. Particularly, when there is a high fine image reading request, the number of pixels must be increased, so that the drive circuit section which operates at a high speed is needed. However, when the thin film transistor as a thin film semiconductor device of the drive circuit section is made of a semiconductor material of a-Si:H, a mobility of the carrier lies within a range from 0.1 to 0.5 cm.sup.2.C.sup.-1.S.sup.-1 and is not enough large. Consequently, there is a limitation in the charge transfer ability. To improve the charge transfer ability, in general, a size of thin film transistor as one of the thin film semiconductor devices is enlarged, the number of drive circuit sections is set to two, or the like. However, a size of photoelectric converting apparatus is consequently enlarged and the manufacturing costs are also increased. Therefore, it is demanded to realize a photoelectric converting apparatus in which the size is not enlarged and the costs are low and which is suitable for miniaturization and has a drive circuit section having an enough high transfer ability.